The present invention primarily relates to subterranean cementing operations. More particularly, the present invention relates to an improved method for measuring the density and settling properties of cement slurries.
Hydraulic cement compositions are frequently utilized in subterranean well completion and remedial operations. In cementing operations carried out in oil, gas and other wells, a hydraulic cement and various additives are mixed with sufficient water to form a pumpable slurry, and the slurry is pumped into a subterranean zone to be cemented.
For instance, in primary cementing operations hydraulic cement compositions are used to cement strings of pipes such as casings and liners in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is then permitted to set in the annular space. Once hardened, the cement composition forms an annular sheath of substantially impermeable cement which supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks or holes in pipe strings and the like.
It is desirable to create a set cement where the density of the slurry remains consistent from the top to the bottom of the cemented formation. However, solid segregation in the cement slurry causes the fluid portion to migrate upward through the slurry after it is placed in the well bore creating an uncemented area in the annulus. The movement of the fluid portion reduces the density consistency of the cemented portion and the existence of the uncemented area makes the cemented portion unstable. The formation of the free fluid is particularly harmful in deviated or horizontal wells because it can collect along the high side of the annulus forming a channel which can contribute to zonal communication or gas migration. This concern is magnified because the driving force for segregation and separation of the slurry is much more prevalent under deviated conditions. Even in vertical wells, pockets of free fluid located near a corrosive water zone can eventually result in casing leaks. Consequently, the capability of a cement slurry to maintain good suspension properties under downhole conditions is critical.
The amount of water that will separate from a cement slurry during the settling process is referred to as the free water. Historically, the free water amount for a particular slurry has been determined utilizing a standard American Petroleum Institute (API) test well known in the art. During this API test, a 9.4 inch sample of the cement slurry is placed into a 1.375 inch diameter column. As the cement slurry sets, a small amount of water separates from the cement slurry and accumulates on the top of the column. The amount of water accumulated is measured after two hours and is the free water for the particular slurry sample being tested. An excessive amount of free water is a symptom of a poor cement and a poor cement slurry.
A major drawback of the API free water test is that the test is usually carried out at room temperature and atmospheric pressure, rather than at the actual temperature and pressures encountered in the well bore environment. Moreover, the free water test cannot be used at temperatures exceeding 190 degrees Fahrenheit. The suspension properties of a cement slurry tend to reduce at higher temperatures. Because the rate at which a cement slurry cures is dependent upon the surrounding temperature and pressure and the free water test is not conducted under conditions existing during actual use, the free water test does not provide an accurate result.
The BP Settling Test was developed by BP International Ltd as an alternative means to perform relative measurements of cement formulation stability at elevated temperatures and pressures. Generally, in the BP Settling Test the cement slurry is allowed to set hard, permitting free water, unconsolidated solids, and the density profile of the set solids to be distinguished and measured. Specifically, a cement slurry is prepared and heated to the Bottom Hole Circulating Temperature (BHCT) in a pressurized consistometer. The BHCT represents the temperature of a circulating fluid at the bottom of the wellbore after several hours of circulation. The slurry is then cooled and transferred into a settling tube where it is allowed to cure at BHCT. The stability of the slurry is determined once the cement has set. First, the height of the set column of cement is measured and compared with the original height to determine the amount of free water and unconsolidated solids that result from slurry segregation. Next, the settling tube is broken and the cement column is cut into different sections. The density along the cement column is then measured by measuring the bulk volume of each section using the Archimedes Principle.
The BP Settling Test has several disadvantages. First, the cement slurry is subject to aeration when being transferred to the settling tube. This aeration of the cement slurry should be minimized during mixing as any entrained air in the cement slurry will be compressed when the cement is pressurized in the curing chamber causing the cement column to shrink. The shrinkage of the cement column due to compression of air will cause errors in the measurement of the slurry stability. To prevent this false indication of settling the slurry should be placed under a partial vacuum before it is placed in the settling tube. This process is expensive and time consuming.
Another shortcoming of the BP Settling Test is that with today's increasing temperatures and pressures of cementing, the BP Settling Test has inherent risks and problems that could lead to false readings of a cement density. Specifically, in the BP Settling Test the sample is heated to the BHCT, removed from the consistometer and cooled down before being transferred to the settling tubes and reheated to the BHCT. These steps are time consuming and hazardous. Moreover, the temperature reduction of the slurry is often accompanied by a viscosification of the slurry as it cools. This viscosification alters the slurry properties and reduces the test's accuracy as there is no such cooling in real downhole conditions.
Yet another disadvantage of the BP Settling test is that once the settling tube is broken, shards of glass have to be carefully removed from the slurry surface before further measurements can be obtained. This process is time consuming and gives rise to safety concerns.